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(Semi)auto combustion?

A place to ask general spud cannon related questions.
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(Semi)auto combustion?

Unread postAuthor: ByeBye » Sun Sep 19, 2010 4:37 am

It works like an ak47: http://www.youtube.com/watch?v=wX3vIW9MkNk
Gas pushing back the piston and reload.
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BOOM!!!
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Darts won't fit in a magazine like thaht. So I'll use cartridges.
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Spring pulls back the bolt and sucks in fresh air.
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(need to find a way to inject propane)
Pushing the trigger ignites the fuel/air mixture. When it is refueled, the bolt returns and ignites it again. Repeats until it runs out of ammo or fuel or when you release the trigger.
Image

Something I need to find out is the position of the gas port on the barrel. If it's too far to the end, it won't have enough gas to reload and when it is too far to the back, it will shoot inefficient. Also need to know the inner diameter of the gas tube.

I also don't if it will work with everything on the right place, maybe it lacks power to operate or something else.

Edit: Pics are not to scale.
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Last edited by ByeBye on Sat Oct 30, 2010 6:03 pm, edited 2 times in total.

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Unread postAuthor: MRR » Sun Sep 19, 2010 4:47 am

A normal combustion doesn't generate enough pressure to cycle a bolt, especially when we are talking about a small caliber dart gun.
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Unread postAuthor: ByeBye » Sun Sep 19, 2010 4:49 am

Well, back to the drawing board

EDIT: New Idea: Bolt action. Manually cycle bolt but still auto refueling and fresh air:
Cycle Bolt -> pull trigger -> BOOM!!! -> Cycle bolt -> pull trigger -> BOOM!!!

No problems about gas port position etc 8)
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Last edited by ByeBye on Sun Sep 19, 2010 5:20 am, edited 3 times in total.

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Unread postAuthor: qwerty » Sun Sep 19, 2010 4:59 am

A normal combustion doesn't generate enough pressure to cycle a bolt, especially when we are talking about a small caliber dart gun.


Did you see the recoil on JSR's combustion cartridge?
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Unread postAuthor: MRR » Sun Sep 19, 2010 5:12 am

@qwerty
Yes I've seen JSRs cartridge prototype but that concept is different to a gas operated blowback piston reloader. The cartridge ejects by its recoil energy while ByeByes idea is based on cycling a mechanical piston with gases that are generated during the combustion.

@ByeBye
That is very possible to work. Have a look at this pump action paintball gun:

Image
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Unread postAuthor: al-xg » Sun Sep 19, 2010 7:22 am

Venturi nozzle for fuel mixing...
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Unread postAuthor: Fnord » Sun Sep 19, 2010 12:43 pm

I don't think you'll get a cycling action with this design, however you gave me a brilliant idea.

You don't need a gas-piston. Make the bolt fairly heavy, and put a spring behind it.

The vacuum that forms after the shot will suck the bolt back.

You won't get every last bit of air out, but you may be able to correct this by putting a 120 psi pop-off valve in the back. A heavy bolt will gather KE when cycling, and may have enough force to pop the valve when it slams back.

Or, alternatively, just make the bolt hit a button-actuated popoff when it is all the way back.
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Unread postAuthor: ByeBye » Sun Sep 19, 2010 2:41 pm

Fnord wrote:I don't think you'll get a cycling action with this design, however you gave me a brilliant idea.

You don't need a gas-piston. Make the bolt fairly heavy, and put a spring behind it.

The vacuum that forms after the shot will suck the bolt back.

You won't get every last bit of air out, but you may be able to correct this by putting a 120 psi pop-off valve in the back. A heavy bolt will gather KE when cycling, and may have enough force to pop the valve when it slams back.

Or, alternatively, just make the bolt hit a button-actuated popoff when it is all the way back.

I don't understand the pop off valve part. Could you explain that to me :D?
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Unread postAuthor: Fnord » Sun Sep 19, 2010 3:11 pm

Ok,

When you react propane with air, it gets hot and expands; typically I think to about 8 times its volume.

When it cools off and pulls a vaccuum, it'll shrink down until the pressure equalizes with the atmosphere. This will mean about 1/8th of your chamber volume worth of waste gasses will be left.

What I'm attempting to do is make the bolt pick up speed from the vaccuum force, then slam into that remaining 1/8th volume of waste gas. If all goes well, the force of the bolt will pressurize the gas enough to make the popoff vent, thereby forcing the last bit of gas from the chamber.
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Unread postAuthor: ByeBye » Sun Sep 19, 2010 3:34 pm

Would the check valve at the back of the bolt do the same job?
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Unread postAuthor: Fnord » Sun Sep 19, 2010 4:17 pm

Yeah, it will.
Duh, I completely forgot about it for some reason.
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Unread postAuthor: ByeBye » Sun Sep 19, 2010 4:41 pm

al-xg wrote:Venturi nozzle for fuel mixing...


I have one from a broken jet lighter. Would it inject the right amount of air and fuel?
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Unread postAuthor: POLAND_SPUD » Sun Sep 19, 2010 5:26 pm

Would it inject the right amount of air and fuel?
It could work for a combustion but not for a hybrid.
DCVs are your best bet if you want to measure out the exact amount of air and fuel for a hybrid

I think he meant that venturi effect might be useful for mixing
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Unread postAuthor: ByeBye » Mon Sep 20, 2010 3:26 pm

Thx to Fnord for the vacuum reloading idea:
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Still one problem: When the air cools down, it sucks back the bolt. I think thats gonna work. BUT I also want it to suck in fresh air when the bolt returns to the front. At pics in the first post you can see the check valve at the back of the combustion chamber, I removed it from these pics because I thought it wouldn't cycle the bolt because the vacuum would also suck air in via the valve at the back of the combustion chamber, resulting in not enough air pushing the bolt to the back.
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Unread postAuthor: Fnord » Mon Sep 20, 2010 3:33 pm

True.
You can get around this by either making the rear intake fairly small to restrict flow, or by not letting it open until the bolt goes back. I can't think of an easy mechanism to do the latter right now because I'm tired. JSR where are you?
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